1. Field of the Invention
The present invention relates to a developing device used for image forming devices such as copying machines, facsimiles, or printers, and to a magnet roller used for the developing device.
2. Detailed Description of the Related Art
Conventionally, there have been known image forming devices, such as copying machines, facsimiles or laser printers, which make visible an electrostatic latent image formed on an image carrying body and which obtain a recorded image. There are developing devices used for such image forming devices which adopt a magnet roller as a developer carrying body. The magnet roller, which carries and transfers a developer by means of magnetic force, is widely used because it has an advantage in that it is stable in view of environmental change and it supports high speed, compared to a non-magnetic roller which carries and transfers the developer using only static electricity. There are two types of developers used for the developing device: a type which uses a magnetic toner, and a type which uses a two-component developer comprised of a magnetic carrier and a non-magnetic toner. The developer is suitably selected according to the developing speed, image quality and the like of the image forming device.
As a representative developing device, there is known the developing device which carries and transfers a two-component developer by means of a developing roller comprised of a rotatable sleeve enveloping inside a fixed magnet (i.e., a magnet roller). In this developing device, the following operations are carried out.
1. (Lift Up) The developer is attracted onto the sleeve by the magnetic force of the magnet. PA0 2. (Transfer) A developer controlling member (hereinafter called a doctor blade) is made to contact the developer on the rotating sleeve or arranged to the developer through a gap to control the amount of developer on the sleeve. PA0 3. (Development) The developer is napped in a developing region opposing an image carrying body, and a static latent image is developed upon the image carrying body. PA0 4. (Recovery) The developer is transferred into the developing device. PA0 5. (Detachment of Developer) The developer is detached from the sleeve, and is returned into the developing device.
In order to carry out the above operations, the developing roller generates a magnetic field alike the one shown in a sectional view of FIG. 3(a). The radial magnetic flux density of the developing roller and the magnetic flux density in the tangent direction thereof are controlled to be in an appropriate range. (Refer to FIG. 3(b).
Further, among these developing devices, in order to realize size reduction, there has been proposed a type, as shown in FIG. 1, which does not comprise a developer stirring member for stirring the developer in the axial direction.
In this type of developing device, the supplied toner and the developer is stirred by the movement of a magnetic carrier caused by the magnetic force of the magnet roller and rotation of the sleeve.
The magnetic field generated by the magnet roller has a magnetic force component in the axial direction. FIG. 4 shows a vector of the axial magnetic flux density of the magnet roller. When the magnet roller has a magnetic force in the axial direction, the movement of the developer differs, even if the radial magnetic flux density of the magnet roller and the magnetic flux density in the tangent direction are the same. The developer is drawn and moved in the axial direction by the axial magnetic force of the magnet roller. Although the developer is moved only slightly, repetitive movement over time will result in a large amount of movement which cannot be ignored, and there occurs a bias, in the axial direction, in the amount of developer being carried or in the concentration of the toner. If such a bias of the developer in the axial direction becomes large, a difference in developing ability will occur, and an uneven image in the axial direction will be formed. Particularly, as shown in FIG. 1, in the developing device which does not comprise a developer stirring member for stirring the developer in the axial direction, since the developer is stirred only by the magnetic force of the magnet roller and the movement of the magnetic carrier caused by rotation of the sleeve, the influence of the axial magnetic force becomes large.
The above-described bias of the developer caused by the axial magnetic force of the magnet roller becomes notable when controlling the amount of the developer on the sleeve with the doctor blade of the developing device. This is because the developer, except for the developer which has passed through the gap between the sleeve and the doctor blade, is not held instantly by the sleeve because it comes into collision with the doctor blade, and the developer is in a condition where it is subjected to only the magnetic force of the magnet roller, as shown in FIG. 2. In such a condition, the developer is easily moved because of the axial magnetic force of the magnet roller. Such repetitive movement of the developer causes a notable bias of the developer in the axial direction.
Further, the bias of the developer, caused by the axial magnetic force of the magnet roller, also occurs in the vicinity of a developing region. In front of the developing region wherein the magnet roller and an image carrying body oppose each other, there is brought about a condition called an agent build-up where the developer having been transferred piles up. The developer in this agent build-up portion is likely to be subjected to an influence caused by the axial magnetic force, compared to the developer held by the sleeve. Thus, the bias of the developer, caused by the axial magnetic force of the magnet roller, is brought about also in the vicinity of the developing region.
The reasons why the magnet roller possesses an axial magnetic force are that: a magnetic circuit is formed at the end portion of the magnet roller, specifically in an area about 30 mm from the end surface thereof, because the magnetic field is turned at the end portion (see FIG. 4); and the direction of ferrite oriented into a resin of the magnet roller forms an angle with reference to the axial direction. The former is inevitable because of the shape of the magnet roller, and is difficult to avoid. The latter is deeply related to a molding step in a manufacturing process of the magnet roller.
A method of manufacturing a magnet roller is such that the roller is molded while orienting ferrite into a resin in the axial direction, generally according to magnetic field-injection molding or magnetic field-extrusion molding. In the case of magnetic-field injection molding, a variation in the orientation of ferrite is caused by the way the resin flows or because of pressure distribution under a pressure-dwelling condition. Among these causes, the pressure distribution is difficult to control; and therefore, in magnetic field-injection molding, it is difficult to keep ferrite from orienting with an angle with reference to the axial direction and to reduce the axial magnetic force of the magnet roller.
As for magnetic field-extrusion molding, a variation in ferrite orientation is caused by resin flow. FIG. 7(a) is a diagram explaining the flow of resin in magnetic field-press molding. A mixture of ferrite 11 and resin 10 is squeezed from a head 12 towards an orientation die 13 for orientating the ferrite 11 (see FIG. 7(b)). FIG. 7(b) is a diagram explaining the flow of resin within the orientation die, and the direction of ferrite. When squeezing the mixture of ferrite 11 and resin 10 from the head 12 towards the orientation die 13, the resin 10 flows from the outside towards the central portion, and because of this flow, at the entrance of the orientation die 13, the resin 10 turns into a parallel flow having a velocity gradient, that is, a shear flow 15. Further, within the orientation die 13, if there is a temperature gradient of the resin 10 or a frictional force with the orientation die 13, these tend to increase the shear flow 15. Because of this shear flow 15, the ferrite 11 in the resin orientates with an angle with reference to the axial direction, and an axial magnetic force component is generated in the magnet roller molded product 16. (See FIG. 8.)